1. Field of the Invention
The present invention relates to a radio broadcast system.
2. Description of the Prior Art
Conventionally, in some communication systems, arrangement have been put into practice to confirm arrival of information which has been broadcast by one base station to a plurality of subsidiary stations located around the base station. Examples of known methods are the polling method in which the base station sequentially inquires of all subsidiary stations about transmission, the ACK ("acknowledgement") method in which subsidiary stations which have properly received information make an answer, and the NAK ("negative acknowledgement") method of requiring data retransmission only in case of transmission failures.
Also proposed are a method wherein subsidiary stations which frequently issue a data retransmission request or make no response are removed from the service, thereby more efficiently broadcasting information to a plurality of subsidiary stations (improving the transmission throughput) (e.g., a method disclosed in Japanese Unexamined Patent Publication No. 63-129743), a method wherein identical data are transmitted a plurality of times to improve the communication quality, thereby making a transmission confirmation procedure such as ACK/NAK unnecessary (e.g., a method disclosed in Japanese Unexamined Patent Publication No. 63-246938).
In these prior art systems, a necessary condition for employment of the NAK method in radio broadcast is that the radio transmission state between the subsidiary stations and the base station must be so good that the transmission of the NAK signal need not be confirmed. If a subsidiary station which does not satisfy this condition is contained in the system, retransmission control is always performed for the subsidiary station, so the throughput of the entire system is lowered. To prevent such a decrease in throughput, generally, a means for removing from the service the subsidiary station which frequently generates an NAK signal, or a means for transmitting identical data a plurality of number of times to improve the communication quality, must be used together. In this case, the probability of reception of the NAK signal by the base station is an important parameter influencing the transmission capability and quality of the system. However, the NAK signal using a coding code defined in the system is conventionally processed like data and does not particularly take a measure for increasing the reception probability. Degradation in the NAK signal reception rate makes the base station erroneously determine that the broadcast data has been properly transmitted to all subsidiary stations. For this reason, the means for optimizing the transmission capability and quality of the system may not be normally selected.